A part of mesh turn into cyan in edit mode
Those edges are marked as sharp. If faces &amp; edges are marked as smooth blender will simulate a smooth edge. Shading is computed from the direction of the edge normals. They will merge into one when smoothed.



If you want rid of the blue lines (yellow in my viewport) select everything and set all edges to smooth. If you want to play around with smooth/sharp faces and edges set all faces to smooth and enable auto-smooth (and perhaps increase the angle to 180). The relevant buttons are marked in red below.